This invention relates to engine exhaust monitoring systems for vehicle mounted internal combustion engines, in particular such systems using oxygen sensors of the solid electrolyte type in the engine exhaust system. Such systems are useful in providing a signal indicative of air-fuel ratio to engine fuel controls in a manner well known in the art.
It is well known and documented in the prior art that the output voltage of such sensors is affected by the sensor temperature. It might be advantageous, therefore, to include, in some closed loop fuel control systems, apparatus for monitoring sensor temperature and generating an output signal in response thereto. For example, in a system relying on the heat of engine exhaust gases to heat the sensor to its normal operating temperature, such monitoring apparatus could compare the sensor temperature signal to a reference and signal when the operating temperature has been reached. In a system with an electrically heated sensor, such monitoring apparatus can provide a sensor temperature feedback control signal to maintain a desired sensor temperature. In addition, it might be possible to use the signal from such sensor temperature monitoring apparatus to compensate the sensor's output signal as its temperature changes.
To be most useful, such temperature monitoring apparatus should not require any changes in the sensor itself, such as the inclusion of extra temperature measuring elements or extra connection terminals. In addition, such apparatus should ideally require no further modifications to the vehicle exhaust system, such as the addition of a temperature measuring element in the vicinity of the oxygen sensor. Instead, the temperature monitoring apparatus should be of the type which connects to the normal output terminal or terminals of the oxygen sensor so that it can be used with any prospective sensor design and, in fact, could be included within the closed loop electronic package. On the other hand, the temperature monitoring apparatus must not affect the operation of the closed loop system in an unfavorable manner by distorting the output signal of the oxygen sensor or interacting with the rest of the closed loop system in an undesirable manner.